Global forest cover is a key indicator of the health of the planet. An intact forest cycles nutrients, regulates climate, stabilizes soil, treats waste, provides habitat, and offers opportunities for recreation. By a conservative tally, these services are worth more than $4.7 trillion, a total equal to one tenth of the gross world product. Forests also supply goods, including food, medicines, and a large array of wood-based products.

Forests worldwide cover some 3.9 billion hectares—almost a third of the earth's land surface excluding Antarctica and Greenland. Though vast, this wooded area is only half the size of forested land at the dawn of agriculture some 11,000 years ago. Most forests are no longer in their original condition, having changed in composition and quality.

Global estimates of forest cover change are difficult to make because of conflicting definitions of what constitutes a forest, lack of satellite and radar data, and unmonitored land use change. The U.N. Food and Agriculture Organization conservatively estimates that the world lost 94 million hectares of forest in the last decade of the twentieth century. This number assumes that developing countries lost 130 million hectares while the industrial world gained 36 million hectares as abandoned agricultural areas returned to forest. The yearly loss of natural forests during this period, which includes deforestation plus the conversion of natural forests to tree plantations, was 16 million hectares—94 percent of which occurred in the tropics.

During the 1990s, Brazil suffered the heaviest loss of forest—23 million hectares. South America as a whole saw net losses of 37 million hectares. In Africa, 52 million hectares were destroyed. Sudan, Zambia, and the Democratic Republic of the Congo account for half of Africa's forest loss. While the United States gained 4 million hectares of forests, Mexico lost over 6 million, although government reports reveal the loss may be even higher. The total net losses for North and Central America were 6 million hectares.

A massive reforestation campaign in China meant the country added an average of 1.8 million hectares each year during this period, largely because bans on deforestation near the end of the decade heightened the country's reliance on plantations and imports of forest products from other nations. In Indonesia, where tree felling destroyed 13 million hectares over the decade, forest loss has accelerated and now averages 2 million hectares each year. Over the decade, forest cover in all of Asia declined by 4 million hectares.

Although FAO data suggest that world forest loss is slowing, deforestation in tropical areas is accelerating, likely exceeding 13 million hectares each year. As tree cutting in many parts of the world accelerates, nearly half of the remaining forests are at risk. The World Resources Institute estimates that about 40 percent of the world's intact forests will be gone within 10-20 years, if not sooner, considering current deforestation rates.

Wood consumption drives deforestation. Since 1960, global industrial wood production has risen by 50 percent, to 1.5 billion cubic meters, four fifths of which is from primary and secondary-growth forests. About the same quantity, 1.8 billion cubic meters, is burned directly as wood fuel each year in developing countries.

Worldwide, only some 290 million hectares of forested land are under protection from logging, but even protected areas are threatened by illegal exploitation. Of 200 areas of high biological diversity worldwide, illegal logging threatens 65 percent. All told, illegal logging has devastated public forests around the globe, reducing incentives for locals to invest in sustainable forestry and accumulating losses of revenue to governments of some $15 billion annually.

Forest plantations now cover more than 187 million hectares, less than 5 percent of total forested area, but account for 20 percent of current world wood production. As natural forests are exhausted or come under protection, a growing share of future wood demand will be satisfied from tree farms.

Well-planned and managed plantations can efficiently satisfy timber demand. Unfortunately, the world has seen many plantations raised at the expense of old growth or other extremely diverse natural forests. In some cases, governments grant forest concessions to logging companies contingent on their planting of replacement trees, but after the companies clearcut, they leave the land bare and move to new areas. In Indonesia, for example, 9 million hectares have been allocated for development as industrial timber plantations, but only 2 million hectares have been replanted.

Areas bereft of their original forest ecosystems and associated habitat have lost vegetation that stabilizes soil, cycles nutrients, and prevents erosion. These lands quickly lose utility and become a liability. Even when plantations are put in place, the functioning of a monoculture plantation is a far cry from that of an old-growth forest, where a number of species of differing ages each play a particular biological role, and ecosystem processes are thus bound to change.

A satellite-based survey of the world's forests by the U.N. Environment Programme, along with NASA and the U.S. Geological Survey, found that 80 percent of largely intact forests (those with a canopy closure of over 40 percent) are located in just 15 countries. A full 88 percent of the key closed forest areas are sparsely populated, making them hopeful targets for conservation. Short of calling for a moratorium of all logging, conservation in these 15 countries offers a reasonable starting point for forest preservation.

Crucial to slowing the loss of the world's natural forests is finding alternative sources of energy for low-income countries, so that valuable wood is not burned. Innovations in reuse and recycling allow reclaimed timber and discarded paper to satisfy wood product demand. Reduced consumption of virgin wood products is a key to saving the world's trees.

When wood products are used, governments can ensure that all domestic production and imports of wood products come from responsibly managed forests meeting rigorous environmental and social standards, like those of the Forest Stewardship Council (FSC). Worldwide, FSC-accredited bodies have certified some 24 million hectares of forests in 45 countries, numbers that are bound to increase as demand for certified wood rises and as noncertified sellers have difficulty competing.